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Life could have survived Earth’s early pounding

By Jeff Hecht

Microbes living deep underground could have survived the massive barrage of impacts that blasted the Earth 3.9 billion years ago, according to a new analysis. That means that today’s life might be descended from microbes that arose as far back as 4.4 billion years ago, when the oceans formed.

Around 3.9 billion years ago, shifts in the orbits of the gas giant planets are thought to have disrupted other objects in the solar system, sending many hurtling into the inner planets. Geologists call that time the Hadean Eon, and thought its fiery hell of impacts would have sterilised the Earth.

But a new study by Oleg Abramov and Steve Mojzsis of the University of Colorado in Boulder suggests hardy life-forms could have survived if they were buried underground. They will report the results on 23 March at the Lunar and Planetary Science Conference in Texas.

Sterilisation point

Using a computer model, they sent 200 million billion tonnes of mass – in rocks with the same mass distribution as those in today’s asteroid belt – slamming into the planet.

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The biggest impacts would have done the most damage – a 500-kilometre-wide blockbuster would have spread a 350-metre-deep layer of 1200 °C ejecta over the planet.

Yet heat from the impacts would not have penetrated very deeply into the underlying solid crust. The layer heated to the sterilisation point, about 110 °C, would be only about 300 metres thick. High-temperature ‘extremophile’ microbes, like those in the hot springs of Yellowstone National Park, would have survived at greater depths, down to their limit of about 4 km.

Moreover, the impacts might have helped provide a refuge for these heat-loving microbes by creating cracks in the rocky crust that water could flow into.

Early oceans

So, how far back could life have originated? It’s hard to say, since rocks that would have recorded evidence of any life from before the Hadean were destroyed by the “late heavy bombardment” that battered the planet at the time. The oldest isotopic evidence of life comes from rocks that formed 3.83 billion years ago, soon after the bombardment ended.

But heat-loving microbes appear to be among the Earth’s earliest life-forms, and may have developed as early as 4.4 billion years ago. That’s when the hot young Earth – whose top few hundred kilometres had probably been vaporised 100 million years before, in the impact that formed the Moon – would have cooled enough for seas to form.

Says Mojzsis&colon; “For all intents and purposes, life could have started 4.4 billion years ago, and the late heavy bombardment pruned, rather than frustrated, life.”

That conclusion is reasonable, says Kevin Zahnle of NASA’s Ames Research Center in California.